An electric traction drive system typically includes a power inverter module (PIM) and an AC motor.
Many common sensorless position control methods of a traction drive systems either rely on spatial variation of rotor saliency of a rotor of the drive system or Back EMF of the inherent saliency machine of the drive system. These methods are more suitable with Interior Permanent Magnet Synchronous Motor (IPMSM), Synchronous Reluctance Motor and Switched Reluctance Motor machine types which inherently have magnetically salient rotors.
Other methods of detecting rotor angular position include high frequency signal injection and modified PWM test pulse excitation.
In the high frequency signal injection method, a balanced high frequency test signal, such as a voltage or current signal, can be injected on a stator winding of an inherently salient machine and the resultant effect of the balanced high frequency test signal on stator current can be measured. The effect of the balanced high frequency test signal injection can be observed in a measured stator current which takes the form of amplitude modulation at two times the fundamental frequency rate. This effect is due to the spatial modulation of the magnetic saliency as the rotor rotates. This method works quite well when the machine under test has inherent saliency, such as an Interior Permanent Magnet type machine. However, Surface Mount Permanent Magnet (SMPM) machines have no saliency and therefore require a very high magnitude injection signal in order to retrieve the position information. Thus, due to additional losses and noise generated by such a high magnitude injection signal, this method is not suitable for SMPM type application.
In the modified PWM test pulse excitation method, modified PWM test pulses can be used to excite the high frequency impedance of the machine. When PWM test pulses are injected, the current control is ignored for the test period. This can be a good method for an industrial drive. However, a traction machine has low inductance and not controlling current during test period may result in an uncontrolled condition. This technique retrieves the position information from sensed stator current which must be sampled immediately after injecting the test pulses. This increases number of times the stator current is being sampled.
According to another method of rotor position estimation discussed in European patent # EP 0962045 B1, the resultant effect of modified PWM test pulses on leakage inductances can be measured via a measured phase to neutral voltage of an induction machine. This technique utilizes the property of a squirrel cage type rotor construction in an induction machine. By injecting the PWM test pulses, the “mechanical” saliency induced due to the rotor bars of the induction machine can be utilized. A saliency image rotates as the rotor rotates and this information can be used to deduce rotor position information.
However, magnetic saliency is not a feature of a Surface Mount Permanent Magnet Motor (SMPMM). Thus, position sensorless control for a SMPMM can be challenging.
A high performance permanent magnet (“PM”) machine drive system requires an absolute position sensor which is an expensive component. Moreover, the circuitry required to process its signals can also be expensive. It would be desirable to eliminate this position sensor. It would also be desirable to eliminate mechanical interface hardware, reduce cost and weight, reduce cost and weight and improve the reliability of an electric traction drive system. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.